1. Field of the Invention
The present invention relates to a method for determining the rotational position of the drive shaft of a commutated direct current (DC) motor by digitizing the armature current signal of the motor and then evaluating the digitized armature current signal.
2. Background Art
The armature current signal of a commutated DC motor includes a direct component and a ripple component superimposed on the direct component. The ripple component arises when the motor is operated as a consequence of the interaction of the magnet (field), the armature winding, and the commutator of the motor. This expresses itself in a transient change in the induced voltage which produces the ripple content in the armature current signal. The current peaks contained in the armature current signal—referred to below as current ripples—occur when the armature of the motor rotates.
The number of current ripples in a full revolution of the armature corresponds to the number of armature collector bars. For example, if the armature has ten collector bars then the armature current signal will have ten current ripples upon a full revolution of the armature. Thus, the number of counted current ripples is indicative of the actual rotational position of the motor's armature and drive shaft. Consequently, the counted current ripples is indicative of the position an element such as a motor vehicle window being driven by the motor along a predetermined travel segment.
In order to count the current ripples, the analog armature current signal is digitized. The number of current ripples counted in a certain time interval is the current ripple frequency. The current ripple frequency is indicative of the actual rotational speed of the motor.
To make it possible for current ripple detection to be performed with as few errors as possible, the analog armature current signal is conditioned before and possibly after digitization in order to suppress interference. Filtering is done to condition the armature current signal. The filtering may be in the form of low-pass filtering and/or frequency filtering.
For example, DE 195 11 307 C1 describes such a signal conditioning process. The purpose of such signal conditioning processes is to provide a precise armature current signal having minimal interference so that the current ripples contained in this conditioned armature current signal can be evaluated. To determine the position of the driven element, the current ripples in the conditioned armature current signal are counted. The counted result provides direct information regarding the actual rotational position of the drive shaft and the motor's armature. The current ripples contained in the armature current signal are usually counted using minima or maxima determination algorithms, or other algorithms to determine the zero crossings.
However, it can happen that the armature current signal contains missed and/or double current ripples which falsify the current ripple counter result. Missed current ripples are current ripples which are not detectable in the armature current signal even though a rotational movement of the motor's armature took place. Double current ripples are current ripples which appear in the armature current signal as double peaks of a single current ripple, so that if both maxima are counted during a maxima count, the current ripple counter result is mistakenly incremented by an extra count.
For appropriate correction of the current ripple counter result when missed and/or double current ripples occur it is common for the signal conditioning and evaluation processes to have a downstream correction process. The correction process is intended to identify the occurrence of missed and/or double current ripples so that it is then possible to make an appropriate correction in the current ripple counter result. The use of such a process is necessary because these errors are caused by the commutator or other superimposed interference, e.g., the ripple content in a vehicle electrical system, and thus they cannot easily be eliminated by conditioning the armature current signal.
DE 197 29 238 C1, for example, discloses such a correction process. In this process, at the time point when a current ripple is detected, the actual value of the rotational motor speed, as determined from the motor current and characteristic data, is used to calculate the point in time when the detection of the next current ripple is expected. This point in time is part of a tolerance band which has a fixed size. Thus, the process disclosed in this document involves enlarging the calculated probable time point of the next commutation (current ripple) by the size of the specified tolerance band. Accordingly, the absence of a current ripple at or before the calculated time point is only identified as a missed ripple if a current ripple also has not been detected within the tolerance band. However, this process is computationally intensive.